Encoding systems for sensing relative movements usually have a row of equidistantly spaced encoder marks attached to a first member and a sensor arrangement attached to a second member which is movable with relation to the first one. When the encoding marks move past the sensor arrangement signals according to the detected marks are generated. By counting the signals, information regarding the position change between the two members (i.e. relative-position information) is obtained.
An example of such a system is a wheel with radial encoder marks mounted on a rotating axle, for example a pulley axle of a belt arrangement in an ink-jet printer. Each time a mark passes an (e.g. optical or magnetic) sensor, a signal is generated which indicates that the wheel has turned by one mark. Another example is a belt of an ink-jet printer equipped with a linear row of encoder marks. Each encoder signal indicates a belt advance of one mark. However, if all marks are identical, no absolute-position information is provided by such an encoder system.
In order to provide absolute-position information, it is known to equip an encoder with at least one index mark (or “reference mark”). Two types are known:
In double-channel (or multi-channel) encoders, the encoding marks and the reference mark are separately arranged and viewed by individual sensors (for example, U.S. Pat. No. 5,206,645, FIG. 3).
In single-channel encoders, the different marks are arranged in one single encoder scale. Examples of such single-channel encoders are, for example, described in U.S. Pat. Nos. 4,786,803, 4,789,874, 5,206,645 and 5,411,340. In these single-channel encoders, the index mark is wider than the encoder marks, and in some of them one or more of the encoder marks are “hidden” by the index mark. The sensor generates different signals for the marks of different widths, which enables the two types of marks to be distinguished. According to U.S. Pat. No. 5,411,340, the virtual position of the hidden encoder mark is estimated.